In one form of process for the production of gases containing hydrogen and carbon monoxide from solid fuels at elevated pressure in a fluidized bed using a gasification agent use is generally made of a gasification reactor which has a lower conical portion in which the fuel to be gasified is put into a fluidized condition by the gasification agent. The fluidized bed which is produced in that way and within which the fuel particles are in a condition of constant movement has an upper and a lower boundary which are normally not sharply defined. The lower boundary is formed by a solid bed which is beneath the fluidized bed and which comprises finer and coarser solid gasification residues which are possibly sintered together. The solid gasification residues such as ash are drawn from the reactor at the lower end of the solid bed.
Together with gas which is produced within the fluidized bed, and any excess gasification agents that may be present, solid particles issue from the upper boundary surface of the fluidized bed, which boundary surface is in a condition of substantial movement. Those fuel particles pass into a generally cylindrical portion of the reactor which extends the conical lower portion thereof in an upward direction and within which a post-gasification space or zone is to be found. One or more gasification agents is or are also introduced into the post-gasification zone in order as substantially as possible also to gasify any fuel particles which are entrained out of the fluidized bed. The fuel particles and the gas produced are also in a condition of vigorous movement in the post-gasification zone, without however all particles falling back into the fluidized bed. On the contrary a large proportion of the particles together with the synthesis gas produced is discharged from the reactor at the upper end thereof, and those particles have to be at least in part separated out of the product gas in at least one separator which is normally in the form of a cyclone separator so that the synthesis gas leaves the separator in a condition of having undergone at least preliminary cleansing.
The solid particles which are separated off in the cyclone separator often still contain so much carbon that it is worthwhile recycling them to the reactor. With an increased feed of gasification agent or agents into the fluidized bed, it is even possible to achieve an operating condition which can be identified as an `circulating fluidized layer`. In that situation, an upper boundary is no longer formed for the fluidized bed. On the contrary, so much gasification agent is introduced that the predominant proportion of fuel particles passes into the post-gasification space and from there into the separator and therefore must be recycled if an adequate degree of gasification effect is to be achieved.
The recycling conduit through which the solid particles which are separated off in the cyclone are returned into the reactor extends between the cyclone separator, more particularly generally between the lower part thereof, and the reactor, with the layout normally being such that the recycling conduit opens into the reactor in the region of the fluidized bed, that is to say, in the lower region of the reactor. In order to bridge over the horizontal spacing which is usually to be found between the separator and the reactor, the recycling conduit extends inclinedly, that is to say at an acute angle with respect to the vertical, at least in parts thereof. At any event the interior of the reactor, the separator and the recycling conduit form a coherent and intercommunicating system.
When recycling the solid materials, difficulties may arise because that system has a pressure drop such that the pressure decreases within the reactor in an upward direction, that is to say in the direction of the flow in the reactor gases. A further pressure drop occurs within the separator, the pressure in the separator in the region adjoining the recycling conduit for the solid material which is separated out of the gas being even lower than the pressure in the upper region of the reactor. On the other hand, at the end of the recycling conduit, remote from the separator, at which the recycling conduit communicates with the lower region of the reactor, the recycling conduit encounters the higher pressure obtaining within the reactor. As a result, different pressures are effective at the two ends of the conduit, with the two regions at the different pressures being more or less effectively screened off relative to each other by the solid material which accumulates in the conduit on its way back to the reactor. In a practical situation, that system involves confused and undefinable operating conditions within the recycling conduit, which have the result that the return flow of the solid material separated off in the separator into the reactor is prevented or at least adversely affected. That can result in a blockage in the recycling conduit as the solid particles therein become clogged up, particularly in the inclinedly extending portion of the conduit. Added to that is the fact that the above-indicated operating condition gives rise to pressure equalization procedures which cannot be monitored and which cannot be influenced and which also result in operational faults and defects and can even adversely affect the separation capability of the separator.
The risk that the solid material in the recycling conduit may form a blockage therein with the result that, after a short period of time, the progressively increasing length of the blockage becomes such that solid material accumulating in the recycling conduit reaches the separator, may also be attributed to the fact that the recycling conduit is of small diameter in comparison with the length thereof. The length of the conduit will generally be determined by the distance to be covered between the separator and the region of the fluidized bed reactor into which the solid material to be recycled is to be introduced. An increase in the diameter of the recycling conduit, which would help in counteracting the danger of blockages occurring, is not a viable proposition, as that would have an undesirable effect on the pressure and flow conditions in the entire installation, more specifically possibly even to such an extent that the system would no longer be operational. It should be appreciated that an increase in the diameter of the recycling conduit which, as already mentioned, generally communicates with the reactor in the lower region thereof, would have the result, with a given reactor diameter, that a larger proportion of the gaseous fluidization agent could pass into the lower region of the recycling conduit, possibly together with solid particles, so that the flow conditions which are intended to provide a direction of flow in the reactor upwardly therein, from there into the communicating conduit lading to the separator and from there by way of the recycling conduit back into the lower region of the reactor, could possibly be reversed or could at any event be subjected to an influence which would exclude orderly operation, with increasing recycling conduit diameter. In other words, having regard to the prescribed parameters involved, the recycling conduit must be of a suitably small diameter with a correspondingly high flow resistance in order to act as a kind of throttle means to prevent a pressure equalization effect from occurring as between the lower portion of the reactor and the separator.
As the absolute magnitude of the pressure drop in the system increases with increasing pressure within the reactor, the effects of the pressure drop on the material which is to be recycled from the separator into the reactor are correspondingly high in modern gasification reactors which are operated at a pressure of 20 bars or more.